In this specification, the term “optical” is given the meaning typically used by those skilled in the art, that is, “optical” generally refers to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infrared and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibers. Further, the term “optical signal” as used in this specification refers both to optical energy that is modulated with an information signal and to optical energy that is not modulated. Still further, “beam” or “optical beam” refers to optical energy that propagates through a free space environment, such as the atmosphere, and optical energy that propagates within dielectric optical waveguides such as optical fibers.
In the fields of optical communication and lasers, it is desirable to be able to compensate for dynamic phase fluctuations introduced into an optical beam by atmospheric turbulence. In general, without some form of adaptive optical processing to compensate for atmospheric or other signal-distorting effects, an optical channel will demand a much greater link power budget to achieve the necessary communication bandwidth at a prescribed bit-error rate.
Previous adaptive optical processing systems typically use one of the following servo-control architectures: (1) hard-wired servo-loops which obtain the necessary wavefront control information from a wavefront sensor (such as a Shack-Hartmann sensor) and then drive an array of multiple phase-control elements; or (2) an all optical approach (IAO (integrated adaptive optics), phase conjugation, real-time holography, etc.) which involves non-linear mixing or holographic processing of the optical wavefront in a material (e.g., multiple quantum well (MQW)) or a pixelated photoconductive surface (a spatial light modulator or electronic holographic camera, etc.).
Both of the aforementioned systems require a means by which to relay wavefront information from the target location back to the transmitter (which contains the optical source and the spatial light modulator). Wavefront information is also known as phase fluctuation information. In the hard-wired approach, phase fluctuation information is received and processed at an optical receiver. The processed information is then coupled back to the multi-pixel phased array via some type of communication cable. Such a technique is useful because phase fluctuations reduce the power received at a receiver. Therefore, by adjusting the phase, improved bit-error rates and optimized link budgets can be achieved. However, such a hard-wired system is impractical for long-haul scenarios (e.g., from one airborne platform to another).
In the all-optical approach, light is reflected from a dominant (glint) feature of a target. The reflected light is then received by a receiver that processes the wavefront information of the reflected light. Because the wavefront information is determined using reflected light, the path through which the light is traveling distorts the light twice, once upon traversal to the target, and once upon return from a dominant (glint) feature on the target. This reduces the overall signal to noise ratio and convergence time of the system. Also, this double pass architecture increases the probability of the system to form up (i.e., converge) on an undesirable feature (perhaps, a more dominate glint) which increases the probability of the interception and detection of the light.
As a result, there is a need for an adaptive optical system that uses a single-pass architecture to compensate for phase fluctuations of an optical beam. There is also a need for an adaptive optical system in which the information for phase fluctuation compensation can be transmitted over long distances.